1. Field of the Invention
The present invention relates to an organic electroluminescence (EL) driving circuit and a passive matrix organic EL display device which can reduce power consumption occurring when a passive matrix organic EL display panel is operated.
The present application claims priority of Japanese Patent Application No. 2000-403533 filed on Dec. 28, 2000, which is hereby incorporated by reference.
2. Description of the Related Art
A passive matrix organic EL display panel is a display panel in which an organic EL element formed by stacking a thin film made up of an organic material and being a micro-light emitting unit containing no active element is placed on a substrate in a matrix form, requiring no backlight and now drawing the attention of people as a spontaneous light emitting type display device. The organic EL element, however, has a large problem. That is, since a parasitic capacity that a light emitting section has is structurally large at a time of a high-speed operation, a charging current of the organic EL element has to be reduced. To solve this problem, some technologies have been proposed (for example, in Japanese Laid-open Patent Application No. Hei 11-143429).
FIG. 6 is a diagram showing an example of configurations of a conventional passive matrix organic EL display device 100. FIG. 7 is a diagram showing a state of connection occurring at a time being different from a time shown in a case of the conventional passive matrix organic EL display device 100 in FIG. 6. FIG. 8 is a diagram showing another state of connection occurring at a time being different from the time shown in the case of the conventional passive matrix organic EL display device 100 in FIG. 6.
The conventional passive matrix organic EL display device 100, as shown in FIG. 6, chiefly includes a passive matrix organic EL display panel in which a plurality of organic EL elements E11, E12, E13, . . . , E1n, E21, E22, E23, . . . , E2n, E31, E32, E33, . . . , E3n, E41, E42, E43, . . . , E4n, . . . , Em1, Em2, Em3, . . . , and Emn is arranged in row and column direction and in a matrix form and in which one terminal of each of organic EL elements E11, E12, . . . , Emn is connected to each of a plurality of scanning lines R1, R2, R3, R4, . . . , and Rm for every row and another terminal of each of the organic EL elements E11, E12, . . . , Emn is connected to each of a plurality of data lines C1, C2, C3, . . . , and Cn for every column, horizontal driving change-over switches 11, 12, 13, 14, . . . , 1m placed on every scanning line R1, R2, . . . , Rm in each row, driving sources 21, 22, 23, . . . , 2n placed in every data line C1, C2, Cn in each column, charging switches 31, 32, 33, . . . , 3n placed in every data line C1, C2, . . . , Cn in each row, a voltage holding circuit 4 placed commonly on an output side of the charging switches 31, 32, 33, . . . , 3n in each column, a first power source 5 and a second power source 6.
The passive matrix organic EL display device 100 shown in FIG. 6 is constructed in a matter that organic EL elements E11, E12, . . . , Emn each corresponding to one of three primary colors made up of red (R), green (G), and blue (B) colors are formed in a form of a strip of paper and the organic EL elements E11, E12, . . . , Emn each having a number corresponding to each of the three primary colors are arranged in a same area and in a same arrangement order and a plurality of sets each including three organic EL elements E11, E12, . . . , Emn each having a different color is arranged on a same substrate so that they make up an pixel for displaying full colors. In the description below, to simplify the explanation, a passive matrix organic EL display panel to display only one color out of the three colors is described.
Each of the organic EL elements E11, E12, . . . , Emn is made up of a diode DE forming a light emitting section and its parasitic capacitor CE and an anode-side terminal of each of the organic EL elements E11, E12, . . . , Emn is connected to each of data lines C1, C2, . . . , Cn and a cathode-side terminal of each of the organic EL elements E11, E12, . . . , Emn is connected to each of scanning lines R1, R2, R3, . . . , and Rm.
The scanning line R1, R2, . . . , Rm in each row is sequentially selected for every scanning cycle and the data line C1, C2, . . . , Cn in each column is sequentially selected in every scanning cycle. Each of the horizontal driving change-over switches 11, 12, 13, 14, . . . , and 1m is, for example, a known semiconductor switch made up of a combination of a P (Positive)-type FET (Field Effect Transistor) and an N (Negative)-type FET, having xe2x80x9cone-pole two-inputxe2x80x9d functions, that is, one port (pole) of the horizontal driving change-over switche 11, 12, . . . , 1m can be connected or switched sequentially to either of other two ports of the same horizontal driving change-over switch 11, 12, . . . , 1m and causes scanning lines R1, R2, . . . , Rm in each row to be connected to a ground when being selected and to be connected to a second power source 6 when being not connected. Each of the driving sources 21, 22, 23, . . . , and 2n feeds an amount of a current corresponding to luminous intensity of light to be emitted while being driven and does not feed the current while being not driven to the data lines C1, C2, . . . , Cn. Each of the charging switches 31, 32, 33, . . . , and 3n, in response to switching operation of the scanning line R1, R2, . . . , Rm on each row, connects a cathode-side terminal of each of the organic EL elements E11, E12, . . . , Emn, in parallel, to an anode-side of the voltage holding circuit 4. The voltage holding circuit 4 includes a constant-voltage element DH made up of a Zener diode (ZD) and parallel capacitor CH having electrostatic capacity being equivalent to a sum of all organic EL elements E11, E12, . . . , Emn making up the passive matrix organic EL display panel and is adapted to hold a voltage on the anode side of all organic EL elements E11, E12, . . . , Emn at a fixed electric potential VH determined by the constant-voltage element DH when each of the charging switches 31, 32, 33, . . . , and 3n is turned ON due to grounding of the cathode-side terminal. The first power source 5 applies a voltage V1 to each of driving sources. The second power source 6 applies a voltage V2 to each of horizontal driving change-over switches 11, 12, . . . , 1m.
Operations of the conventional passive matrix organic EL display device 100 will be described by referring to FIGS. 6, 7, and 8.
FIG. 6 shows a state in which the scanning operation is switched from a scanning line R1 in a first column to a scanning line R2 in a second column and the scanning line R2 is connected to a ground through the horizontal driving change-over switch 12. At this point, cathodes of all organic EL elements being connected to the selected scanning line R2 are connected to a ground. For example, when the data line C2 is in a driving state and when a driving current is fed from the first power source 5 through the driving source 22, in the organic EL element E22 being connected between the data line C2 and the scanning line R2 and now shown by being circled by a broken line, the fed driving current causes the diode DE to emit light with intensity corresponding to an amount of the fed driving current and also causes the parasitic capacitor CE to be charged.
Each of the organic EL elements being connected to the selected scanning line R2 and being connected to each of the data lines C1, C3, . . . , Cn but being not driven does not emit light, since each of corresponding driving sources 21, 23, . . . , 2n feeds the driving current to a degree which causes each of the organic EL elements to be a voltage level being less than a light emitting threshold value (hereinafter the voltage level being referred to as a xe2x80x9cblack levelxe2x80x9d). A voltage at which the organic EL element reaches the black level differs depending on a light emitting color. On the other hand, each of the organic El elements being connected to each of scanning lines R1, R2, . . . , Rm being not selected does not emit light since a voltage having a same polarity as that of the first power source 5 is applied from the second power source 6 to the cathode-side of each of the organic EL elements and therefore each of the organic EL elements is put into a reverse-biased state in which a reverse-directional voltage is applied to each of their diodes. At this point, the parasitic capacitor CE of each of the organic EL elements is charged so as to be in a state of the reverse biased potential.
FIG. 7 shows an initial state in which the scanning is performed on a scanning line R3 in a third column with subsequent timing, that is, in which each of the charging switches 31, 32, 33, . . . , and 3n is turned ON and the scanning line R2 is connected to the second power source 6 through the horizontal driving change-over switch 12 and the scanning line R3 is connected to a ground through the horizontal driving change-over switch 13. At this point, all the data lines C1, C2, C3, . . . , and 3n are connected each other through the charging switches 31, 32, 33, . . . , and 3n, which, as a result, are all connected to the anode-side of the voltage holding circuit 4. Then, an electric charge flows from the organic EL element which was driven and emitted light at the previous time and, as a result, all other organic EL elements are charged and voltages on their anode-side are held at the fixed electric potential VH determined and fixed by the voltage holding circuit 4. The fixed electric potential VH is a voltage at which the organic EL element with its cathode being connected to a ground reaches the black level, which causes all the organic EL elements being connected to the selected scanning line R3 to be pre-charged so as to be at the black level.
FIG. 8 shows a state in which each of the charging switches 31, 32, . . . , 3n is turned OFF and setting of the potential using the voltage holding circuit 4 has completed. At this point, all the data lines C1, C2, C3, . . . , and Cn are separated from each other and each of the data lines is separated from the voltage holding circuit 4. Moreover, since the scanning line R2 is connected to the second power source 6, the voltage on the cathode-side of the organic EL element E22 is raised to the level of the second power source 6 and, as a result, the organic EL element E22 is put into a reserve-biased state and its light goes off.
On the other hand, by the connection of the scanning line R3 newly selected to the ground, the driving current is fed from the driving line C2 to the organic EL element E32 existing on a next row and, as a result, the organic EL element E32 emits light with intensity corresponding to an amount of the fed driving current and the parasitic capacitor CE is charged. Moreover, the current at the black level flows through organic EL elements 31, E33, . . . , E3n being connected to the scanning line R3 newly selected but not being driven from the driving sources C1, C3, . . . , and Cn. At this point, since the parasitic capacitor CE of the organic EL element E32 has been charged so as to be at the black level determined by the voltage holding circuit 4 with the previous timing, an amount of electric charges to be applied before a start of light-emitting to the parasitic capacitor CE of the organic EL element E32 required at a time of being newly selected may be smaller, compared with a case in which a cathode of the organic EL element is connected to a ground at a time of being not selected, which enables emitting of light with high intensity in the organic EL element E32.
In the passive matrix organic EL element display device 100 shown in FIGS. 6, 7, and 8, since the organic EL element being connected on a newly selected scanning line and being driven has been already charged, with its previous timing, to a voltage of the charge holding circuit 4, an amount of the electric charge required before light is emitted is small and, therefore, there is an advantage in that high-speed light emitting is achieved.
However, the conventional passive matrix organic EL element display device 100 has a problem. That is, since the parasitic capacitors CE of the organic EL elements not being selected are all charged, at every time of switching of the scanning line, at a voltage being equivalent to a difference between a voltage of the second power source 6 and that of the voltage holding circuit 4 and, as a result, current consumption of the entire device increases, causing power source capacity to be larger.
In view of the above, it is an object of the present invention to provide an organic EL driving circuit and a passive matrix organic EL display device capable of reducing charging currents being produced at a time of switching of scanning lines and to be supplied to an organic EL element being connected to a scanning line being not selected.
According to a first aspect of the present invention there is provided an organic electroluminescence driving circuit for driving a passive matrix organic electroluminescence display panel in which a plurality of organic electroluminescence elements is arranged in row and column directions in a matrix form and in which one terminal of each of the organic electroluminescence elements is connected to each of a plurality of scanning lines in every row and another terminal of each of the organic electroluminescence elements is connected to each of a plurality of data lines in every column, the organic electroluminescence driving circuit including:
a plurality of driving sources each being placed on every data line in each column and each feeding a driving current from a first power source to a data line selected at every scanning timing,
a plurality of charging switches each being placed on every data line in each column and each connecting all data lines to a voltage holding circuit at an initial stage of the scanning timing and releasing the connection at an end stage of the scanning timing,
a voltage holding circuit to hold each of the connected data lines at a fixed voltage; and
a plurality of horizontal driving change-over switches each being placed on every scanning line in each row and each connecting selected scanning lines at an initial stage of the scanning timing to a ground and, at the end stage of the scanning timing, each connecting the selected scanning line to a second power source and, in a subsequent scanning cycle and thereafter, each performing switching so as to cause the selected scanning line to be in a high impedance state until the scanning line is again selected next.
In the foregoing, a preferable mode is one wherein the fixed voltage held by the voltage holding circuit is a voltage corresponding to a black level of the organic electroluminescence element.
Also, a preferable mode is one wherein the voltage holding circuit is made up of a constant voltage element which holds the fixed voltage and an electrostatic capacitor which is connected in parallel to the constant voltage element.
Also, a preferable mode is one wherein the voltage holding circuit is made up of a constant voltage source which generates the fixed voltage.
According to a second aspect of the present invention, there is provided an organic electroluminescence driving circuit for driving a passive matrix organic electroluminescence display panel in which a plurality of organic electroluminescence elements is arranged in row and column directions and in a form of a matrix and in which one terminal of each of the organic electroluminescence elements is connected to each of a plurality of scanning lines in every row and another terminal of each of the organic electroluminescence elements is connected to each of a plurality of data lines in every column, the organic electroluminescence driving circuit including:
a plurality of driving sources each being placed on every data line in each column and each feeding a driving current from a first power source to the data line selected in every scanning cycle;
a plurality of charging switches each being placed on every data line in each column and each operating to connect all the data lines to a ground at an initial stage of the scanning cycle and releasing the connection at an end stage of the scanning cycle; and
a plurality of horizontal driving change-over switches each being placed on every scanning line in each row and each operating to connect selected scanning lines at an initial stage of the scanning timing to a ground and to connect the selected scanning line to a second power source at an end stage of the scanning timing and, in a subsequent scanning cycle and thereafter, to perform switching so as to cause the selected scanning line to be in a high impedance state until the scanning line is again selected next.
In the foregoing, a preferable mode is one wherein the second power source has a voltage enough to cause all the organic electroluminescence elements being connected to the selected scanning line to be in a reverse-biased state.
Also, a preferable mode is one wherein the second power source has a same voltage as that of the first power source.
According to a third aspect of the present invention, there is provided a passive matrix organic electroluminescence display device including:
a passive matrix organic electroluminescence display panel in which a plurality of organic electroluminescence elements is arranged in row and column directions and in a matrix form and in which one terminal of each of the organic electroluminescence elements is connected to each of a plurality of scanning lines in every row and another terminal of each of the organic electroluminescence elements is connected to each of a plurality of data lines in every column, the organic electroluminescence driving circuit including:
a plurality of driving sources each being placed on every data line in each column and each feeding a driving current from a first power source to the data line selected in every scanning cycle;
a plurality of charging switches each being placed on every data line in each column and operating to connect all the data lines to a ground at an initial stage of scanning cycle and to release the connection at an end stage of the scanning cycle;
a voltage holding circuit to hold each of connected data lines at a fixed voltage;
a plurality of horizontal driving change-over switches each being placed on every scanning line in each row and each operating to connect selected scanning lines to a ground at an initial stage of the scanning timing and at an end stage of the scanning timing to connect the selected scanning line to a second power source at an end state of the scanning timing and, in a subsequent scanning cycle and thereafter, to perform switching so as to cause the selected scanning line to be in a high impedance state until the scanning line is again selected next.
In the foregoing, a preferable mode is one wherein the fixed voltage held by the voltage holding circuit is a voltage corresponding to a black level of the organic electroluminescence element.
Also, a preferable mode is one wherein the voltage holding circuit is made up of a constant voltage element to hold the fixed voltage and an electrostatic capacitor connected in parallel to the constant voltage element.
Also, a preferable mode is one wherein the voltage holding circuit is made up of a constant voltage source to generate the fixed voltage.
According to a fourth aspect of the present invention, there is provided a passive matrix organic electroluminescence display device including:
a passive matrix organic electroluminescence display panel in which a plurality of organic electroluminescence elements is arranged in row and column directions and in a matrix form and in which one terminal of each of the organic electroluminescence elements is connected to each of a plurality of scanning lines in every row and another terminal of each of the organic electroluminescence elements is connected to each of a plurality of data lines in every column;
a plurality of driving sources each being placed on every data line in each column and each feeding a driving current from a first power source to the data line selected in every scanning cycle;
a plurality of charging switches each being placed on every data line in each column and operating to connect all the data lines to a ground at an initial stage of the scanning cycle and to release the connection at an end stage of the scanning cycle; and
a plurality of horizontal driving change-over switches each being placed on every scanning line in each row and operating to connect selected scanning lines to a ground at an initial stage of the scanning timing and to connect the selected scanning line to a second power source at an end stage of the scanning timing and, in a subsequent scanning cycle and thereafter, to perform switching so as to cause the selected scanning line to be in a high impedance state until the scanning line is again selected next.
In the foregoing, a preferable mode is one wherein the second power source has a voltage enough to cause all organic electroluminescence elements being connected to the selected scanning line to be put in a reverse-biased state at an end stage of the scanning timing.
Also, a preferable mode is one wherein the second power source has a same voltage as that of the first power source.
According to a fifth aspect of the present invention, there is provided a driving method of a passive matrix organic electroluminescence display panel in which a plurality of organic electroluminescence elements is arranged in row and column directions and in a matrix form and in which one terminal of each of the organic electroluminescence elements is connected to each of a plurality of scanning lines in every row and another terminal of each of the organic electroluminescence elements is connected to each of a plurality of data lines in every column, the display panel provided with a horizontal driving change-over switch on the scanning line in each row used to switch a state of selected scanning lines among a grounding state, high-voltage applying state, and high-impedance state the driving method including:
a step of, at an initial stage of scanning timing, connecting the selected scanning line to a ground and putting the organic electroluminescence element being connected to the scanning line into a state where it is able to be driven in the column direction;
a step of connecting, after end of a driving period, the selected scanning line to a high voltage applying power source and causing all the organic electroluminescence elements being connected to the scanning line to be put in a reverse-biased state;
a step of performing switching so as to cause the selected scanning line to be put into a high impedance state until the scanning line is again selected next, in a subsequent scanning cycle and thereafter.
With the above configurations, in the organic EL driving circuit and passive matrix organic EL display device, since the second power source is connected only to the scanning line which has just completed the scanning operation in order to cause the organic EL element to be reverse-biased and since the second power source is not connected to any other scanning line, an amount of the charging current being produced when the second power source is connected to the scanning line and flowing between the organic EL element and the voltage holding circuit becomes equal only to that of currents flowing through the parasitic capacitor of the organic EL element being connected to the selected scanning line. As a result, no unnecessary charging currents flow through the parasitic capacitor of all the organic EL elements being connected to the scanning line being already in the non-selected state and, therefore, it is possible to reduce current consumption largely compared with the conventional device adapted to cause all the scanning lines in a non-selected state to be reserve-biased, which enables a reduction of power consumption of the passive matrix organic EL display device and scaling-down of the display device.
Moreover, the horizontal driving change-over switch used to select the scanning line in the passive matrix organic electroluminescence display panel is so constructed to have the xe2x80x9cone-pole three-inputxe2x80x9d function and, at an initial stage of scanning timing, the selected scanning line is connected to a ground and, at an end stage of the scanning timing, the selected scanning line is connected to the second power source. As a result, the scanning line being not selected is put into a floating state and, therefore, the power source used to cause the organic electroluminescence element to be reverse-biased is connected only to the scanning line which has just completed its scanning operation and other scanning lines are kept in an high-impedance state and an amount of the charging current being produced when the second power source is connected to the scanning line and flowing between the organic EL element and the voltage holding circuit becomes equal only to that of currents flowing through the parasitic capacitor of the organic EL element being connected to the selected scanning line. As a result, no unnecessary charging currents flow through the parasitic capacitor of all the organic EL elements being connected to the scanning line being already in the non-selected state and, therefore, it is also possible to reduce current consumption required to cause the organic EL element being not selected to be reverse-biased.